A Fractal-Based Correlation for Time-Dependent Surface Diffusivity in Porous Adsorbents

Fluid–solid adsorption processes are mostly governed by the adsorbate transport in the solid phase and surface diusion is often the limiting step of the overall process in microporous materials such as zeolites. This work starts from a concise review of concepts and models for surface transport and variable surface diusivity. It emerges that the phenomenon of hindered surface diusion for monolayer adsorption, which is common in zeolites, and models able to fit a non-monotonic trend of surface diusivity against adsorbate solid phase concentration, have received limited attention. This work contributes to the literature of hindered diusion by formulating a time-dependent equation for surface diusivity based on fractal dynamics concepts. The proposed equation takes into account the contributions of both fractal-like diusion (a time-decreasing term) and hopping diusion (a time-increasing term). The equation is discussed and numerically analyzed to testify its ability to reproduce the possible dierent patterns of surface diusivity vs. time.